Two independent openings of the BBB were demonstrated following one hour MCA occlusion in cats. The first opening occurred shortly after release of occlusion and was associated with high elevations of the rCBF. The second opening of the barrier was demonstrable after 5 hours following release of occlusion and was associated with severe ischemic tissue changes. Both openings of the barrier were dependent on the rCBF falling below threshold values (12 ml/100/g/Min) during the occlusion.